Conventionally, an injection molding machine is known which comprises an injection mechanism for melting and injecting a material to be formed by injection molding such as, for example, a thermoplastic material or a thermosetting resin (referred to simply as "resin", hereinafter) by means of a heating cylinder which is provided with a screw (and/or a plunger), a mold clamping mechanism for clamping a metal mold with a high pressure, and a control mechanism for automatically operating these mechanisms, and which operates in such a manner that the resin is dropped from a hopper into the heating cylinder to be melted and mixed and kneaded, the resin thus heated and molten is forced (injected) into the metal mold at a high pressure by the forward movement of the screw and, after being cooled and solidified, is then released from the mold, thereby obtaining molded products.
When the resin which is melted by heating is injected into the metal mold, a part of the molten resin existing close to a surface of the metal mold (that is, a layer of resin which forms an outer portion of a molded product) 1 is cooled rapidly, while another part of the molten resin existing in the inner part of the metal mold (that is, a layer of resin which forms an inner part of the molded product) 2 is cooled gradually, as shown in FIG. 4, thereby causing shrinkage of the resin.
On the other hand, as shown in FIG. 5, the injection pressure 3 is maximized immediately after the commencement of the injection and decreases sharply upon the completion of filling of the mold with the resin (or when changed over to a hold pressure). To the contrary, the internal pressure 4 in the metal mold becomes sharply reduced when the molded product is taken out (or when the cooling is completed) although it is very high immediately after the completion of the filling of the mold with the resin. In short, the molten resin is subjected to very complicated variations in operations of parameters in that it contracts due to the compressing action of high injection pressure applied immediately after the commencement of the injection and it expands due to the decompressing action of the respective pressure mentioned above.
In this way, the molten resin suffers a difference in volume caused due to its complicated behavior mentioned above, or what is called "molding shrinkage", in the course of the injection molding operation. This molding shrinkage of the resin results in generation of surface depressions which seem to occur most frequently among defects of the molded products.
FIG. 6 is a sectional view for explanation of the principle of the generation of such depressions. In FIG. 6, a reference numeral 5 denotes a depression, and a numeral 6 denotes a part of the resin which was cooled rapidly, which forms the outer portion of the molded product and which exhibits the same temperature change as that of the part of molten resin 1 existing close to the surface of the metal mold shown in FIG. 4. A reference numeral 7 denotes another part of the resin which was cooled gradually and forms the inner portion (or the inner part of a large-thickness portion) of the molded product and which exhibits the same temperature change as that of the part of molten resin 2 existing in the inner part of the metal mold (or the layer of resin which forms the inner portion of the molded product) shown in FIG. 4 as well. The depression 5 is caused to occur by such a phenomenon that the part of resin 7 which was cooled gradually pulls down the part of resin 7 which was cooled rapidly.
In short, the depression referred to above is caused by the molding shrinkage resulting from the very complicated behavior of the molten resin as mentioned before and is accordingly considered to be a defect which has been very difficult to eliminate.
In order to prevent such depressions, there have hitherto been taken various measures such as, for example, a measure in which the molding is performed under conditions such that the injection pressure is increased and the temperature in the injection heating cylinder is lowered while the hold pressure is applied sufficiently; a measure in which the diameters of a sprue and a runner, particularly of a gate, are increased; and a measure in which the molded product and the metal mold are designed, so that it will be difficult for a depression to occur.
However, it is hard to say that every prior measure is a satisfactory and effective depression preventing measure because they are attended with technical difficulties and require annoying control means.
Incidentally, although not directly related to a depression preventing measure, a molding method is known in which a foaming agent is added to the resin to form the molded products of spongy plastics such as, for example, urethane foam and foam polyethylene. According to this method, the occurrence of depressions can be prevented due to increased pressure produced when foaming. However, since such foaming conventionally takes place even in the surface layer, the appearance of the molded product is not as good as would be desired.
Further, what is called a "sandwich molding method" is also known in which a first resin, for forming the surface layer, is first introduced into the mold under pressure and then another resin (combined with a foaming agent), for forming the inner part of the molded article, is introduced into the mold under pressure. According to this method, however, the necessity of provision of two molding machines makes the structure complicated, and the application of this method is limited to the forming of the molded products having a large thickness.
Accordingly, an object of the present invention is to provide an injection molding method for forming molded products of foamed plastics which is capable of preventing the occurrance of depressions and of neatly finishing the appearance of the molded products by making use of a novel measure that is entirely different from the conventional measures in which an excessively high injection pressure and hold pressure are applied.